1. Field of the Invention
This invention relates to a method and device for ultrasonic cleaning which efficiently separates and removes radioactive solids such as crud or scale adhering to the various members, in particular hollow square-shaped members, which make up a light-water reactor atomic power plant by irradiating them with ultrasonic waves.
2. Discussion of the Background
One example of a light-water reactor atomic power plant is a boiling water reactor (hereinbelow abbreviated as BWR). Typically this is constructed as follows. A reactor pressure vessel contains a reactor core and cooling water. The reactor core consists of a plurality of fuel assemblies and control rods etc. The cooling water flows upwards over the core and is heated by the heat of the nuclear reaction of the core. The heated cooling water assumes a two-phase flow condition consisting of water and steam and is introduced into a steam/water separator arranged above the core, where the water and steam are separated. The separated steam is further passed into a steam drier arranged above the separator, where it is dried to produce dry steam. This dry steam is supplied for power generation by being fed to a turbine system through a main steam pipe connected to the reactor pressure vessel After being used in the turbine to generate electricity, the steam is fed to a condenser where it is condensed, liquefied and returned to condensate. The water which was separated in the steam/water separator flows down through a downcomer and is mixed with the feedwater returned from the turbine system and fed to a location below the core. The above cycle is then repeated.
In an atomic power plant, corrosion products generated by corrosion of the various pipes and equipment etc. which make up the atomic power plant are the main cause of radiation exposure. These corrosion products acquire their radioactivity by being irradiated by the fuel assemblies, to which they adhere when they are transported to the fuel assemblies by the cycle described above. Some of these irradiated corrosion products then separate from the fuel assemblies and become suspended in the coolant water in the reactor or are dissolved, etc. and dispersed in the atomic power system so as to adhere to the pipes and equipment, raising the proportion of radioactivity in the atmosphere. This results in radioactive exposure when workers enter this atmosphere. Removal of such radioactive crud or scale adhering to the fuel assemblies and various items of equipment is therefore very effective in greatly reducing the amount of exposure to radioactivity in an atomic power plant. Removal of this radioactive crud is also important in the case of fuel, from the point of view of preventing dispersal of radioactive pollutants during handling, when moving spent fuel out into spent fuel storage installations or into nuclear fuel reprocessing plants etc. Equipment onto which radioactive corrosion products adhere can be classified into square-shaped hollow items such as fuel racks, which hold the fuel assemblies, and cylindrical items such as pipes. This description is concerned in particular with a cleaning technique for removing corrosion products adhering to square-shaped hollow items.
Taking a fuel assembly as an example of a square-shaped hollow item, a prior art fuel assembly cleaning device will now be described. One example is a water-spray cleaning device as disclosed in issued Japanese Patent Publication Sho. (Tokko-Sho) 58-17440. This device will be described with reference to FIG. 1 and FIG. 2. FIG. 1 is a view showing the overall layout of the entire device. In FIG. 1, a wash chamber 1 is illustrated. This wash chamber 1 is of an elongate cylindrical shape such as to surround a fuel assembly 2 and spray nozzle head 3. As shown in FIG. 2, a spray nozzle head 3 is equipped with a square-section through-hole 4 matching the shape of fuel assembly 2, the fuel assembly 2 being inserted within this through-hole 4. A plurality of spray nozzles 5 are mounted on the inner circumference of through-hole 4. After removing the channel box, high pressurized water is sprayed onto fuel assembly 2 through the plurality of spray nozzles 5. Spray nozzle head 3 is mounted such that it can be raised and lowered along wash chamber 1. The construction of a drive unit which carries out this raising and lowering action is described below. A motor 8 is arranged on a floor 7 above a fuel pool 6, gearing 9 being coupled to a rotary shaft of this motor 8. This gearing 9 is coupled to a screw bar 11 by means of a swivel joint 10. A nut 12 mounted on spray nozzle head 3 is threaded onto this screw bar 11. Reference numeral 13 denotes a guide bar for ensuring that spray nozzle head 3 is driven vertically. Thus, when motor 8 is started up, its rotation is reduced by gearing 9, its transmission direction is converted, and it is transmitted to screw bar 11 through swivel joint 10. By rotation of this screw bar 11, spray nozzle head 3 is raised and lowered vertically, by means of a nut 12, while being guided by guide bar 13.
A water feed unit is connected to spray nozzle head 3 and highly pressurized water is fed from this water feed unit. In more detail, a water feed pump 14 is arranged on floor 7 and pool water 6b in fuel pool 6 is sucked in through suction pipe 15 by this water feed pump 14. Pool water 6b which is sucked in is fed to each nozzle 5 of spray nozzle head 3 through a blowdown hose 17 so that highly pressurized water can be sprayed from these nozzles onto fuel assembly 2.
A drainpipe 19 is arranged at the bottom 6a of fuel pool 6. Reference numeral 19 in FIG. 1 denotes a centrifugal separator. Centrifugal separator 19 and the bottom of wash chamber 1 are connected through a manifold 20. An underwater vacuum pump 21 is inserted in this manifold 20. A crud receiver 24 is connected through outlet nozzle 22 and a remotely operated disconnective joint 23 to a lower portion of centrifugal separator 19. In FIG. 1, reference numeral 25 indicates an opening, and 26 indicates a support which supports the fuel assembly 2 from below. Pool water 16 containing crud which flows out from below fuel assembly 2 is fed into centrifugal separator 19 where it is separated into clean pool water and a solid fraction (separated crud). The pool water 16 is discharged through opening 25 into fuel pool 6 while the solid fraction is collected in crud receiver 24.
However, a fuel assembly cleaning device constructed as discussed above is subject to the following problems:
(1) Since the high pressurized water is sprayed from outside the fuel assembly within wash chamber 1 under a condition wherein the channel box removed from fuel assembly 2, or with a fuel assembly which originally does not have a channel box mounted within wash chamber 1, the highly pressurized water is prevented from penetrating into the interior because it is obstructed by the fuel rods outside the fuel assembly. This prevents crud adhering to fuel rods which are located on the inside of the fuel assembly from being removed.
(2) A large amount of highly pressurized water is needed in cleaning fuel assembly 2. This means that water feed pump 14 and/or underwater vacuum pump 21 which is used for removing the water which is supplied are very large in size and thus difficult to handle. Furthermore anxieties exist regarding being able to guarantee the necessary installation space and regarding contamination of these devices themselves, leading to the concern that large amounts of radioactive waste etc. may be generated.
(3) During the cleaning process the channel box has to be mounted and removed. This complicates the operation and increases exposure of the workers. Furthermore there are safety problems due to the increased possibility of damaging the fuel rods since fuel is manipulated with the channel box removed.
(4) The amount of solids adhering to the fuel tends to be decreased in modern plants thanks to management of water quality etc. but adhesion is stronger. This means that one cannot expect to obtain the same cleaning efficiency as in conventional plants.
Another practical example using ultrasonic waves will now be described. Examples of fuel cleaning using ultrasonic waves are: Early Japanese Patent Publication Sho. (Tokkai-Sho) 55-104799, Early Japanese Patent Publication Sho. (Tokkai-Sho) 59-58399, Early Japanese Utility Model Publication Sho. (Jitsukai-Sho) 60-113600 and the publication entitled "Feasibility of Using Nonchemical Methods to Decontaminate Fuel Rods": EPRI NP-4122, June 1985. EPRI NP-4122 will now be described with reference to FIG. 3 and FIG. 4. Reference numeral 31 in FIG. 3 denotes a wash chamber. A fuel assembly 32 and ultrasonic transducer 33 are disposed within this wash chamber 31. Fuel assembly 32 and ultrasonic transducer 33 are arranged parallel to each other, so that the ultrasonic waves are incident at right angles on the surface of the fuel assembly. An ultrasonic generator 34 is connected to ultrasonic transducer 33 by means of a cable 37. Ultrasonic transducer 33 can be raised and lowered along a guide 36 by means of a translating mechanism 35. That is, ultrasonic waves are directed onto fuel assembly 32 while raising and lowering the ultrasonic transducer 33, thereby removing crud adhering to the fuel rods. A filter 39 is connected to the foot of wash chamber 31 through a drainpipe 38. A pump 41 is connected to this filter 39 through pipe 40. Delivery pipe 42 of this pump 41 is connected to the top of wash chamber 31.
With the above construction, radioactive crud adhering to fuel assembly 32, constituting a square-shaped article to be cleaned, is removed while raising and lowering the ultrasonic transducer 33. The crud which is removed flows down together with the pool water and is conducted to filter 39 through drainpipe 38. The crud present in the pool water is removed by filter 39 and cleaned pool water is returned into wash chamber 31 from the top through pump 41 and delivery pipe 42.
An ultrasonic cleaning device constructed as above is subject to the following problems:
(1) In the case of this device also, the channel box is to be removed from fuel assembly 32 or cleaning is to be carried out with a fuel assembly which does not have a channel box. Thus, as in the case of the prior art water jet device described above, a complicated operation is inevitable and there is a risk of damaging the fuel rods.
(2) The ultrasonic transducer is arranged in the wash chamber together with the fuel assembly so the ultrasonic transducer is contaminated by radioactive substances, etc. and so comes to constitute radioactive waste.
(3) Optimum conditions for ultrasonic irradiation are not considered so that high cleaning efficiency is not possible.
Furthermore, regarding cleaning of the fuel rack, which is a square shaped hollow item, methods are employed such as removing solids adhering thereto by spraying with high pressure water, by inserting water jet nozzles in the same way as with the fuel assembly on the inside surface of the tube into which the fuel is inserted. However, as in the case of fuel assembly cleaning, large quantities of highly pressurized water are required. This means that equipment such as pumps has to be made of a large size, making it difficult to manipulate and posing problems regarding installation space and contamination of the device itself, as well as raising concerns that a large quantity of radioactive waste will ensue.
As described above, with a water jet system, crud adhering to fuel rods which are positioned on the inside of the hollow square-shaped item constituted by a fuel assembly is not removed. Furthermore, because the cleaning is accompanied by an operation to remove and refit the channel box before and after cleaning, this requires a large amount of time and there is a risk of damaging the fuel during removal and refitting of the channel box. The large size of the equipment is also a concern. Also in the case of a cleaning device using ultrasonic waves, removal and refitting of the channel box is considered necessary, and, since the ultrasonic transducer is arranged within the water chamber in which the fuel assembly constituting the hollow square-shaped article to be cleaned is placed, it becomes contaminated, creating a problem of radioactive waste disposal.